Congressional Research Service. April 3, 2013.
Recent laws, often called “Amazon laws” in reference to the large Internet retailer, represent fresh attempts by the states to capture taxes on Internet sales. States enacting these laws have used two basic approaches. Until Congress decides otherwise, physical presence remains the standard used to determine the constitutionality of states’ “Amazon laws.” Both the Colorado and New York laws have been challenged on constitutional grounds. Colorado’s notification law appears to be the more constitutionally problematic approach, it was recently struck down by a federal district court. So far, New York click-through law has been upheld.
http://www.fas.org/sgp/crs/misc/R42629.pdf [PDF format, 16 pages].